I have group of students. First I want to group them by the marks. Then I want to further group those sets into same name students together.
Map<Integer,Map<String,List<String>>> groupping = students.stream()
.collect(Collectors.groupingBy(Student::getMarks,
Collectors.mapping(Student::getName,Collectors.toList())));
I am getting an error saying,
Non-static method cannot be refered from a static context.
Yes. I am pretty much aware that I cannot refer to a non-static method without having an instance. But with all these stream operations, I'm a bit confused about what has gone wrong really.
Rather than how to fix this; I really want to know what's going on here. Any of your inputs is appreciated!
Because If I write the below grouping is completely valid;
Map<Integer,List<Student>> m = students.stream().
collect(Collectors.groupingBy(Student::getMarks));
Here is my Student.java class (In case if you need it)
public class Student {
private String name;
private int marks;
// getters, setters, constructor and toString
}
Unfortunately, the error message “Non-static method cannot be refered from a static context.” is just a place-holder for any type mismatch problem, when method references are involved. The compiler simply failed to determine the actual problem.
In your code, the target type Map<Integer, Map<String, List<String>>> doesn’t match the result type of the combined collector which is Map<Integer, List<String>>, but the compiler didn’t try to determine this (stand-alone) result type, as the (nested) generic method invocations incorporating method references requires the target type for resolving the method references. So it doesn’t report a type mismatch of the assignment, but a problem with resolving the method references.
The correct code simply is
Map<Integer, List<String>> groupping = students.stream()
.collect(Collectors.groupingBy(Student::getMarks,
Collectors.mapping(Student::getName, Collectors.toList())));